Help me solve a riddle

[Nyyt]

I have a 2009 honda fit nearing 127,000 miles. Up until about 115,000 miles it didn't burn any oil, it now burns a little, nothing serious maybe a half quart every 3000 miles but I still check it once a week to be on the safe side. It's been fed high quality gas, pennzoil platinum 5w-20 or pennzoil ultra platinum 5w-20 with OCI around 7500-9000 miles with a mobil 1 110a filter. Current fill is Mobil 1 EP 5w-20 and a mobil 1 110a filter with about 1500 miles on it before the trip.

I usually average 34-36 mpg on freeway and 28-31 in the city so nothing exciting other than a 42mpg trip back in the first year I bought it.

I went on a trip this last week about 700 miles one way so 1400 miles total. Went over a few passes, through a couple rainstorms but nothing too out of the ordinary.

Now the riddle, on the last leg of the trip home I hit a whopping 43 mpg in spite of going over a mountain pass and it only producing a 36mpg run there.

To destination
First leg 36mpg -elevation average 2000 feet
Second 33mpg - elevation average 3800 feet
Third 36mpg - elevation average 4200 feet

Return
First Leg 38mpg
Second Leg 34 mpg
Third Leg 43 mpg

On the return trip I used the same gas stations I used on the way over except at the destination which I used the same brand as the others. Tires were at 35 psi, temperature was roughly the same except some rain on third leg to my destination. The only thing I did differently is before the third leg home I noticed it was a half quart low (full at the beginning of the trip) so I put a half quart of the Schaeffers Moly EP treatment as a means to test if a thicker oil would be better without spending 30 dollars for 5w-30 and have to dump it right away if it didn't work. Last year the same thing happened, it was a half to 3/4 of a quart low on a pennzoil ultra 5w-20 fill, topping it off with the same oil I ended up with a 39mpg 60% city/40% freeway run so I don't think it was the schaeffers that did anything.

I have torque pro on my android phone and haven't gone through the logs yet but I'm still stunned at hitting 43mpg on a leg where I expected 35-38mpg.

I'm not complaining, just wondering how this could have happened.

 

[Triple_Se7en]

I experienced similar once, on a 250 trip home to Detroit from Toronto. Had a plus in MPG that blew my mind, considering that this trip was the first and only time I ran 15W40, instead of 5W30 in my 1984 Pontiac 6000 with the 2.5 four-banger. I recall getting around 33 MPG on the way home, which was several MPG over the norm. This was right around 1988.

Moderate wind mostly at my tail, plus much more descents the way home, versus going from Detroit to Toronto. That's my excuse and not the much heavier oil.

 

[ragtoplvr]

With small tank small fill up errors are significant. Lets say you drove 400 miles on 10 gallons of gas for 40 MPG. Now, lets say because of altitude, fuel temp and pump accuracy, you only put in 9.5 42.5 MPG. Then you get a dishonest retailer, and yes they do exist more than you think. You think you put in 10.5 gallons although the pump delivered only 10. 38 MPG.

Then there are fuel differences. When the flooding happened we started getting the no alcohol premium in my area from a different refinery source. My motorcycle went from 43 MPG to 47. Also same result on the MPG display on the on board computer which has always been VERY accurate. I started driving 240 miles per thank, before I would have run out. So I very confident the volume delivered is correct. Went from 43 to 47-48 range. As things go back to normal, I expect my mileage will too.

Rod

 

[CKN]

Unbelievable............

 

[Nyyt]

I would tend to agree with you about the retailer or ethanol but two different times topping off the oil produced an amazing mileage run especially since the two stations are 300 miles apart and the closest one I get gas there so often I haven't seen it crop up before. I've done tests of ethanol vs non ethanol gas in my car, only a 1-2 mpg difference at most, usually about the same in city.

 

[opus1]

Good question. I have had similar experiences when I was making a yearly trek to Minnesota.

I'd stop for gas once I got past Wisconsin Dells and then fill up again when I hit the Twin Cities. Two different vehicles and both of them had 'best ever' mileages on that stretch of the drive. Never come up with a good answer.

 

[DGXR]

Whatever is causing the new oil consumption may also be reducing friction inside the engine... looser tolerances? Just a guess.
I'd be concerned about the motor suddenly going from no oil consumption to 1 quart every 6,000 miles.

 

[Cujet]

Sometimes, mountain driving and higher altitudes can result in improved MPG.

The mountain driving can be much like the pump and glide method of hypermiling. Higher altitudes result in lower engine pumping losses, as the throttle plate is open more, for a given output.

Ethanol content can be a factor.

 

[Nyyt]

I agree DGXR, it might have started earlier and more gradually to be honest as I wasn't as diligent about oil level checking the last couple years up until a few months ago, had a lot of medical issues so car maintenance wasn't foremost on my mind.

 

[Nyyt]

True Cujet, at 2500 feet there is a 1% difference in oxygen so less fuel required for the 14:1 ratio and if there was no ethanol in that batch of gas could give a 1-2 mpg change so I could see 40mpg. Here's a thought, the first leg I filled up at sea level whereas on the way back I filled up at 2500 feet early in the morning for the same leg, would altitude affect volume like temperature does? https://www.ic.gc.ca/eic/site/mc-mc.nsf/eng/lm00129.html shows the temperature density difference. If so it might make up for the last bit of difference I saw. Only thing is it explains this one leg of the trip, it doesn't explain when last year I saw the same thing of topping off the oil. I guess it's nagging at me because I hate not knowing why something happened, especially such large jump. Makes me wonder if something is wrong with the car even though everything looks fine. I appreciate everyone helping me figure this out. Maybe i won't figure it out but still a fun thought exercise.

 

[deanm11]

Do you have or can you get the specifics of the exact temperatures of all the runs/legs? Some cars can be very temp sensitive. Not meant as a thread hijack- maybe our similar experience is on point:

We had a very similar experience when our '12 Subaru Impreza was newer. Very hard to get above 36mph highway. I find the car's fuel economy to deteriorate substantially more than other cars with increasing highway speed and with variations in temperature. The sweet spot would be 50mph and something around 70 degrees. We had one such perfect run that achieved something like +3-4mph compared to all but maybe one other run in-between. We got basically an indicated and hand-calculated 40mph for ~230 mile run back from my cousin's home at edge of Cape Cod, back to our home in Northern NJ. This despite some traffic and rolling hills. I think the traffic worked out to be a wash, as the stop and go was offset by restraint on our speed. Temps must have just hit the perfect zone. I think maybe one other time did at least a car computer indicated 38mph on flat highway more toward PA or interior states.. otherwise I don't think ever anything indicated above 36.x ( or hand calculated.)

 

[dbias]

Coming "downhill" on the return trip? Was going to your destination going west, or into the wind, and coming back going east with wind at your back?

 

[i6pwr]

Even though the last leg was the same route you took at the start of your trip, perhaps it included a lot of downhill coasting then followed by highway miles?

The last leg seems to have the highest change in elevation, assuming this was going downhill?

Were you traveling at a slower speed on the final highway leg.

When you filled up last to calculate the 43 MPG, was it hot outside?

I could be wrong, but isn't the fuel more dense when cold, also less oxygen content..maybe filing up at altitude when ambient temp is possibly cold, then starting downhill followd by highway miles....add all that up and maybe that's your 5 mpg increase.

 

[OilUzer]

wind can be a major factor.
I get 3 or 4 mph difference over one of the passes going E vs. W.

 

[InhalingBullets]

Ethanol and the lack thereof could explain the +/- 10% mileage difference.

 

[Fawteen]

Originally Posted by Nyyt
I'm not complaining, just wondering how this could have happened.


There are a lot of factors that can impact fuel mileage. Wind, amount of time going uphill vs. downhill (even minor grade differences), ambient temperature, minor fuel differences, etc.

We drive to the condo in Orange Beach a couple times each month. Most of the time I get 22 MPG in the pickup both directions. On occasion I'll get up to 24 one direction, and I have also seen as low as 19 on the same route.

Temperature, wind and humidity are the biggest factors.

We also drive up to Huntsville on occasion. The trip there is usually around 21 MPG, the return trip is always higher, about 23 MPG. The biggest difference? Elevation. It's a slow rise to Huntsville, but also a long grade downhill from Huntsville.

 

[Nyyt]

Temp on the first leg there and the last leg home were the same roughly about 74-76. The first leg started at roughly sea level (Seattle area) to Coeur D'Alene which is about 2200 feet above sea level according to my gps. I bought the fuel in the morning on all but the 2nd leg of my trip back. Someone mentioned wind, on the way to CDA there was some wind but didn't feel it pushing whereas on the same leg back it seemed like the air was dead calm, you'd pass a semi and you wouldn't feel the normal drop in wind. Maybe everything was just right, no wind, perfect temperature, non ethanol gas, etc. If anyone is interested I'll pull the logs from my Torque app, I had it running and logging everything the entire way so it might give us a clue.

 

[Nyyt]

I pulled the logs from Torque Pro - you can find it here, I took out the gps location information but the rest of the data is there... TripLog